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    SubjectRe: nohz problem with idle time on old hardware
    On Wed, 13 Nov 2013, Steven Rostedt wrote:
    > > I'm not saying that we are actually getting into nohz, but something
    > > with the nohz code is messing with cpu accounting.
    >
    > The trace does indeed show that a tick is happening, as the config has
    > HZ=250 (4ms) and we see a tick happen every 4ms. But for some reason,
    > we don't update the the idle time correctly when nohz is enabled.
    >
    > When I say nohz is enabled, I mean that we don't have nohz=off in the
    > command line. There seems to be some difference between having nohz=off
    > and having nohz disabled at runtime.

    Right that affects tick_nohz_enabled

    Two files use this variable:
    kernel/rcu/tree_plugin.h
    kernel/time/tick-sched.c

    The only accounting related stuff is in tick-sched.c:

    get_cpu_idle_time_us() and get_cpu_iowait_time_us()

    Both functions bail out if (!tick_nohz_enabled).

    The users of get_cpu_idle_time_us() are cpufreq and fs/proc/stat.c!

    Now the simplest fix is to let those functions check whether we
    actually switched into NOHZ mode. Should work for the RCU tree stuff
    as well.

    Thanks,

    tglx





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